What you need to do is move up your line of counters and make it a solid line from one side of the board to the other. The empty space between your tables and the counters are how they're getting through. So long as there is an open square, the bots will wander right past an open table so you'll want to block things completely. So long as you place an item, any item, on the empty spaces between the tables and your open area your bots will stay where you want.
If you don't want a solid line of counters/deco making an obvious divider between your seating and decorative areas, you can just block the spaces where the bots can walk. Specifically in your current set up, you'll have to place an item on the square after the final chair in each row and then two items in the larger open area to the right of the final row as I've illustrated below with green squares.